1. Field of the Invention
This invention relates to optical add and drop multiplexers (OADMs) used in optical communication systems and more specifically to a compact and inexpensive integrated single-channel OADM with improved optical isolation.
2. Description of the Related Art
Optical communication systems are a substantial and fast-growing constituent of communication networks. Such optical systems include, but are not limited to, telecommunications systems, cable televisions systems, and local area networks (LANs). Wavelength division multiplexing (WDM) is one approach for increasing capacity.
A WDM system employs multiple optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system, optical signal channels are generated at the different channel wavelengths, multiplexed to form a multiplexed optical signal, transmitted over a single fiber or waveguide and demultiplexed such that each channel wavelength is individually routed to a designated receiver.
In many such applications, there is a need to route one or more of the multiplexed channels to different destinations. Such routing occurs when optical channels are sent to or withdrawn from an optical transmission link, e.g. for sending optical channels between a terminal and an optical bus or routing long distance telecommunications traffic to individual cities. This form of optical routing is generally referred to as “add-drop multiplexing.”
To enable a given channel wavelength to be branched off a transmission link using WDM or a given channel wavelength to be added to the link, an element called an optical add/drop multiplexer (OADM) has been developed. The task of the OADM is (1) to direct off a selected narrow-band channel wavelength from the multiplexed optical signal that passes in an input fiber (drop function) and/or 2) to add to an output fiber a narrow-band channel wavelength (add function). The signals (channel wavelengths) that have not been selected pass through the OADM from the input to the output fiber.
As shown in FIG. 1, an OADM 10 receives a multiplexed optical signal from an input fiber 12 and transmits the multiplexed optical signal to an output fiber 14. Typically, most of the signals pass through the OADM 10 with no change. In the case of a 3-port add OR drop multiplexer, the OADM 10 can either remove a signal via drop fiber 16 or replace a signal via add fiber 18. The device includes either drop fiber 16 or add fiber 18 but not both. In the case of a 4-port add AND drop multiplexer, the OADM 10 has the capability to remove one or more of the multiplexed signals from the input fiber 12 and put them on a drop fiber 16 and place replacement signals from an add fiber 18 onto the output fiber 14. Multi-channel OADMs can be made by combining single-channel add AND drop multiplexers or single-channel add OR drop multiplexers.
As shown in FIG. 2, in a typical 3-port single-channel drop OADM 50 the multiplexed optical signal enters OADM 50 through an input fiber 52, which is mounted in a dual-fiber glass ferrule 54 along with output fiber 56. The optical signal exits fiber 52 at the output side of ferrule 54, is collimated by lens 58, suitably a C or GRIN lens, and free-space coupled to a thin-film filter 60. The filter is designed to transmit a particular channel wavelength and reflect the remaining channel wavelengths. The filter specifications depend on what application the OADM is being used for, such as dense or coarse WDM, tap monitoring, multi-channel splitting, etc. In a drop configuration, the transmitted and collimated optical channel wavelength enters a second lens 62, which focuses the signal into a drop fiber 64 in a single-fiber glass ferrule 66. The remaining channel wavelengths are reflected back through lens 58, which focuses the optical signal onto output fiber 56. In the add configuration, all directions of light propagation are reversed. A channel wavelength enters through add fiber (also 64), is collimated by lens 62, passes through filter 60 and enters lens 58, which focuses the signal into output fiber 52 with the multiplexed optical signal.
This implementation is used by JDS Uniphase (Fixed Add/Drop Modules), Oplink Communications (OADM 101A/D Series), Browave (3-Port Add/Drop Filter) and Koncent (Three-Port CWDM). The current 3-port optical add OR drop multiplexer uses two fiber collimators—two sets of lenses, ferrules and fibers, thus has high cost and large packaging size, approximately 5.5 mm in diameter and 32 mm in length. The input/output fibers and add/drop fiber are arranged on opposite sides of the device, which is inconvenient because of the space that is taken up by having fiber bend radii on both sides of the device.
The typical and most practical single-channel OADMs are configured by fiber splicing two 3-port single-channel OADMS together. As shown in FIG. 3, a 4-port OADM is configured by fiber splicing the output fiber 56′ of the 3-port drop OADM 50′ to the input fiber 52″ of a 3-port add OADM 50″. Add OADM 50″ is identical to drop OADM 50′ except that input and output fibers shown in FIG. 2 have been reversed to facilitate the add function. In operation, a multiplexed optical signal carrier on input fiber 52′ enter OADM 50′ and the desired channel is dropped on drop fiber 64′. The remaining optical channels are reflected back to output fiber 56′, which is fiber spliced to input fiber 52″. The multiplexed optical signal, absent the drop channel, enters OADM 50″. An add channel at the same channel wavelength as the drop channel is inserted through add fiber 64″ into the multiplexed optical signal, which is then transmitted on output fiber 56″.
Although adequate, this 4-port OADM uses two identical filters and four fiber collimators—four sets of lenses, ferrules and fibers, thus has high cost and large packaging size because of the length of the devices, the splice, and the fibers exiting both ends of the units. In addition, the out-of-band signals which are reflected by the filters and pass from the input 52′ to the output 56″ experience high insertion loss because they must be coupled into fibers twice before exiting the system
A known 4-port single-channel OADM uses two three-port optical circulators with a narrowband fiber grating (U.S. Pat. No. 5,822,095) or a thin film filter (U.S. Pat. No. 5,926,300) positioned between the two optical circulators. These devices are very expensive and bulky on account of the two three-port circulators and suffer from poor optical isolation. Another single-channel OADM arranges two identical thin-film filters in parallel planes to reflect the input beam twice to solve the optical isolation problem (U.S. Pat. No. 5,812,291). U.S. Pat. No. 5,712,717 proposes another way to improve optical isolation by combining a thin-film filter with a Bragg grating filter through fibers.